1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine having a three-way catalytic converter in an exhaust system thereof.
2. Description of the Related Art
An exhaust system of an internal combustion engine is usually provided with a catalytic converter for converting harmful materials in exhaust gas into harmless materials. A three-way catalyst is generally used as the catalyst, and the three-way catalyst oxidizes CO and HC, and deoxidizes NO, so that these three harmful materials in exhaust gas are converted into CO.sub.3, H.sub.2 O, and N.sub.2, which are harmless materials. Such a purifying ability of the three-way catalyst depends on an air-fuel ratio of the mixture in an engine cylinder, and it is known that when the air-fuel ratio is stoichiometric, the three-way catalyst can purify all of these three harmful materials at the same time. Accordingly, in a conventional engine having the three-way catalytic converter, an air-fuel ratio detector for engine cylinder is arranged in the exhaust passage upstream of the catalytic converter. On the basis of output of the detector, when the air-fuel ratio of mixture becomes higher than the stoichiometric air-fuel ratio, i.e., when the mixture becomes lean, the amount of fuel fed into the engine is increased, and when the air-fuel ratio of mixture becomes lower than the stoichiometric air-fuel ratio, i.e., when the mixture becomes rich, the amount of fuel fed is reduced.
In such a conventional air-fuel ratio feedback control, during transient driving conditions, the air-fuel ratio of the mixture cannot always be made the stoichiometric air-fuel ratio. For example, in an acceleration driving condition, the amount of intake air increases suddenly so that the mixture is temporarily maintained on the lean side, and in a deceleration driving condition, conversely, the mixture is temporarily maintained on the rich side. However, it is known that the three-way catalyst has an O.sub.2 storage function such that is absorbs and stores excess oxygen existing in the exhaust gas when the mixture is on the lean side, and it releases oxygen when the mixture is on the rich side, whereby, the three-way catalyst has a relatively high purifying ability even during these transient driving conditions.
Note, there is a limit to the amount of oxygen which can be stored in the three-way catalyst, so that, to utilize the above O.sub.2 storage function effectively in case of next acceleration or deceleration, it is preferable that the three-way catalyst always stores a predetermined amount of oxygen. Japanese Unexamined Patent Publication No. 3-217633 discloses an air-fuel ratio control device for this purpose. In this device, the output varying against time (it is "0" when the mixture is stoichiometric) from the above air-fuel ratio detector is integrated, and the integrated value is used as the amount of oxygen stored in the three-way catalyst, and the air-fuel ratio control is carried out so as to maintain the integrated value constant.
However, the above integrated value cannot correctly represent the amount of oxygen stored, because the amount of exhaust gas varies every moment particularly during transient driving conditions. Accordingly, in the above device, the amount of oxygen stored in the three-way catalyst cannot be maintained on a predetermined amount. Moreover, the above air-fuel ratio control taking account of only the amount of oxygen stored in the catalyst, requires a relatively long time for the air-fuel ratio of the mixture to converge to the stoichiometric air-fuel ratio. This is undesirable in the engine.